TreeMap do not have “null” keys. Meanwhile different keys are related to various null values.

TreeMap is sorted according to the natural ordering of its keys and it is sorted in ascending order of its keys.

TreeMap contain only unique elements.

TreeMap implementation is not synchronized. Because if map access multiple threads concurrently then one of the threads should be accessed externally using SortedMap sm = Collections.synchronizedSortedMap(new TreeMap).